Optical disk devices utilize laser light for recording data onto and sensing data from storage media. These devices are often used for the storage of computer-prepared data and have value in their ability to store large quantities of data on a single disk. The disk media for use in such devices is reactive to light and is heated thereby to levels which enable the recording of data. To write data on optical media a laser beam is focused onto the media surface and the laser is operated at a relatively high power level in order to alter the media in accordance with an input data stream. In reading data back, the laser power level is controlled to a lower level so that the media is not altered by the laser beam but the reflected light indicates the presence or absence of previously recorded media alterations.
The surface of optical disk media is manufactured with closely spaced grooves, either spiral or concentric. Data recorded on the media is recorded within the grooves. Unalterable identification headers are also included within the grooves. These headers contain information about the disk and enable the system to perform properly. They include identification information for each track so that the system can locate itself on the groove (track) to be written or read.
In order to maintain the laser beam in proper focus on the surface of the optical disk, a focus servo system is provided to move the lens as needed to maintain focus. Once the proper track has been found so that read and write operations can commence, a tracking servo system is operated to maintain the position of the laser beam on the correct track.
One well known source of disturbance in the focus servo system is termed "optical feedthrough" and is due to the inability of the tracking servo to maintain perfect tracking. That is, if the focused laser spot is exactly in the bottom of the groove, and if that position is continually maintained as the disk rotates, no disturbance to the focus is created. However, should the tracking drift slightly, the laser spot moves from the bottom to the sides of the groove and then, under the influence of the tracking servo, back to the bottom and perhaps then to the other side of the groove. The uneven surfaces of the groove create a difference in the light reflection patterns, thus causing changes in the focus error signal. The tracking error signal is also a function of this same movement, thus relating the tracking error signal (TES) to the optical feedthrough feedback into the focus error signal (FES). There are many schemes devised to correct for and eliminate the effect of optical feedthrough.
In addition to the coupling of the focus servo loop and the tracking servo loop by optical feedthrough, some disk drives may also be subject to mechanical resonating conditions at certain frequencies. If mechanical resonance is present, movement of the lens in the focus direction creates a movement in the tracking system because the two are mechanically coupled. When the tracking system is disturbed, a further disturbance occurs in the focus system due to optical feedthrough. Further disturbance in the focus system creates a further disturbance in the tracking system, etc., and the entire system becomes unstable.
It is the object of this invention to provide electronic means for correcting the optical and mechanical system interactions that produce unstable servo systems.